Methods of selectively separating CO2 from a multicomponent gaseous stream

ABSTRACT

Methods are provided for the selective removal of CO 2  from a multicomponent gaseous stream to provide a CO 2  depleted gaseous stream having at least a reduction in the concentration of CO 2  relative to the untreated multicomponent gaseous stream. In the subject methods, the multicomponent gaseous stream is contacted with CO 2  nucleated water under conditions of selective CO 2  clathrate formation, where the CO 2  nucleated water serves as liquid solvent to produce a CO 2  clathrate slurry and CO 2  depleted gaseous stream. In a preferred embodiment, the CO 2  clathrate slurry is then decomposed to produce CO 2  gas and nucleated water. The subject methods find use in a variety of applications where it is desired to selectively remove CO 2  from a multicomponent gaseous stream, including chemical feedstock processing applications and air emissions control applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of application Ser. No.08/923,172, filed on Sep. 4, 1997 now amended, which application is acontinuation of application Ser. No. 08/643,151 filed on Apr. 30, 1996,now U.S. Pat. No. 5,700,311, the disclosures of which are hereinincorporated by reference.

INTRODUCTION

1. Field of the Invention

The field of this invention is selective absorption of CO₂ gas.

2. Introduction

In many applications where mixtures of two or more gaseous componentsare present it is often desirable to selectively remove one or more ofthe component gases from the gaseous stream. Of increasing interest in avariety of industrial applications, including power generation andchemical synthesis, is the selective removal of CO₂ from multicomponentgaseous streams.

An example of where selective CO₂ removal from a multicomponent gaseousstream is desirable is the processing of synthesis gas or syngas. Syngasis a mixture of hydrogen, carbon monoxide and CO₂ that is readilyproduced from fossil fuels and finds use both as a fuel and as achemical feedstock. In many applications involving syngas, the carbonmonoxide is converted to hydrogen and additional CO₂ via the water-gasshift process. It is then often desirable to separate the CO₂ from thehydrogen to obtain a pure H₂ stream for subsequent use, e.g. as a fuelor feedstock.

As man made CO₂ is increasingly viewed as a pollutant, another area inwhich it is desirable to separate CO₂ from a multicomponent gaseousstream is in the area of pollution control. Emissions from industrialfacilities, such as manufacturing and power generation facilities, oftencomprise CO₂. In such instances, it is often desirable to at leastreduce the CO₂ concentration of the emissions. The CO₂ may be removedprior to combustion in some cases and post combustion in others.

A variety of processes have been developed for removing or isolating aparticular gaseous component from a multicomponent gaseous stream. Theseprocesses include cryogenic fractionation, selective adsorption by solidadsorbents, gas absorption, and the like. In gas absorption processes,solute gases are separated from gaseous mixtures by transport into aliquid solvent. In such processes, the liquid solvent ideally offersspecific or selective solubility for the solute gas or gases to beseparated.

Gas absorption finds widespread use in the separation of CO₂ frommulticomponent gaseous streams. In CO₂ gas absorption processes thatcurrently find use, the following steps are employed: (1) absorption ofCO₂ from the gaseous stream by a host solvent, e.g. monoethanolamine;(2) removal of CO₂ from the host solvent, e.g. by steam stripping; and(3) compression of the stripped CO₂ for disposal, e.g. by sequestrationthrough deposition in the deep ocean or ground aquifers.

Although these processes have proved successful for the selectiveremoval of CO₂ from a multicomponent gaseous stream, they are energyintensive. For example, using the above processes employingmonoethanolamine as the selective absorbent solvent to remove CO₂ fromeffluent flue gas generated by a power plant often requires 25 to 30% ofthe available energy generated by the plant. In most situations, thisenergy requirement, as well as the additional cost for removing the CO₂from the flue gas, is prohibitive.

Accordingly, there is continued interest in the development of lessenergy intensive processes for the selective removal of CO₂ frommulticomponent gaseous streams. Ideally, alternative CO₂ removalprocesses should be simple, require inexpensive materials and low energyinputs. For applications in which it is desired to effectively sequesterthe separated CO₂, of particular interest would be the development ofalternative CO₂ absorbent solvents from which the absorbed CO₂ would nothave to be subsequently stripped prior to sequestration. Of particularinterest would be the development of a system which minimizes parasiticenergy losses.

3. Relevant Literature

Patents disclosing methods of selectively removing one or morecomponents from a multicomponent gaseous stream include: U.S. Pat. Nos.3,150,942; 3,838,553; 3,359,744; 3,479,298; 4,253,607; 4,861,351;5,387,553; 5,434,330; 5,562,891 and 5,600,044.

Reports summarizing currently available processes for reducing the CO₂content of mult-component gaseous streams, such as coal fired powerplant emissions, include: Smelser, S. C. et al., "Engineering andEconomic Evaluation of CO₂ Removal From Fossil-Fuel-Fired Powerplants,Vol. 1: Pulverized -Coal-Fired Powerplants," EPRI IE-7365 Vol. 1 andVol. 2; Coal Gasification-Combined Cycle Power Plants, EPRI IE-7365,Vol. 2.

Other publications discussing CO₂ clathrate formation include Japaneseunexamined patent application 3-164419, Nishikawa et al., "CO₂ ClathrateFormation and its Properties in the Simulated Deep Ocean," EnergyConvers. Mgmt. (1992) 33: 651-657; Saji et al., "Fixation of CarbonDioxide by Clathrate-Hyrdrate," Energy Convers. Mgmt. (1992) 33:643-649; Austvik & L.o slashed.ken, "Deposition of CO₂ on the Seabed inthe Form of Clathrates, " Energy Convers. Mgmt. (1992) 33: 659-666;Golumb et al., "The Fate of CO₂ Sequestered in the Deep Ocean," EnergyConvers. Mgmt. (1992) 33: 675-683; Spencer, "A Preliminary Assessment ofCarbon Dioxide Mitigation Options," Annu. Rev. Energy Environ. (1991)16: 259-273; Spener & North, "Ocean Systems for Managing the GlobalCarbon Cycle," Energy Convers. Mgmt. (1997) 38 Suppl.: 265-272; andSpencer & White, "Sequestration Processes for Treating MulticomponentGas Streams," Proceedings of 23^(rd) Coal and Fuel Systems Conference,Clearwater, Fla. (March 1998).

SUMMARY OF THE INVENTION

Methods are provided for the selective removal of CO₂ from amulticomponent gaseous stream. In the subject methods, a multicomponentgaseous stream comprising CO₂ is contacted with CO₂ nucleated waterunder conditions of selective CO₂ clathrate formation, conveniently in areactor. The CO₂ nucleated water may either be formed in situ in thereactor or in a separate reactor, where the water may be fresh or saltwater. Once the CO₂ nucleated water is formed, it serves as a selectiveCO₂ liquid solvent. Upon contact of the gaseous stream with the CO₂nucleated water, CO₂ is selectively absorbed from the gaseous stream bythe CO₂ nucleated water and concomitantly fixed as CO₂ clathrates toproduce a CO₂ depleted multicomponent gaseous stream and a slurry of CO₂clathrates. The resultant CO₂ depleted multicomponent gaseous stream isthen separated from the CO₂ clathrate slurry, either in the reactoritself or in a downstream separator. In a preferred embodiment, theresultant slurry is then treated in a manner sufficient to decompose theCO₂ hydrates to produce CO₂ gas and CO₂ nucleated water. The process issuitable for use with a wide variety of multicomponent gaseous streams.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 provides a schematic representation of an embodiment of acountercurrent reactor for practicing the subject invention.

FIG. 2 provides a schematic representation of an embodiment of aconcurrent reactor for practicing the subject invention.

FIG. 3 provides a flow diagram schematic of a preferred embodiment ofthe subject invention.

FIG. 4 provides a more detailed flow diagram schematic of the CO₂clathrate slurry heat exchanger of the preferred embodiment representedin FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Methods for selectively removing CO₂ from a multicomponent gaseousstream are provided. In the subject methods, a multicomponent gaseousstream is contacted with CO₂ nucleated water under conditions ofselective CO₂ clathrate formation, conveniently in a reactor. The CO₂nucleated water may be prepared in situ in the reactor, or in a separatereactor, where the water may be either fresh or salt water. Upon contactof the gaseous stream with the CO₂ nucleated water, CO₂ is selectivelyabsorbed from the gaseous stream by the CO₂ nucleated water andconcomitantly fixed in the form of the CO₂ clathrates. Contact resultsin the production of a CO₂ depleted gaseous stream and a slurry of CO₂clathrates, which are then separated. In a preferred embodiment, the C0₂clathrate or hydrate slurry is treated to decompose the CO₂ hydrates toproduce CO₂ gas and CO₂ nucleated water. The subject invention finds usein the treatment of a variety of multicomponent gaseous streams.

Before the subject invention is further described, it is to beunderstood that the invention is not limited to the particularembodiments of the invention described below, as variations of theparticular embodiments may be made and still fall within the scope ofthe appended claims. It is also to be understood that the terminologyemployed is for the purpose of describing particular embodiments, and isnot intended to be limiting. Instead, the scope of the present inventionwill be established by the appended claims.

In this specification and the appended claims, the singular forms "a,""an," and "the" include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

Critical to the subject invention is the use of CO₂ nucleated water as aliquid solvent to selectively absorb the gaseous CO₂ from themulticomponent gas phase stream. The CO₂ nucleated water employed in thesubject invention comprises dissolved CO₂ in the form of CO₂ hydrate orclathrate precursors, where the precursors are in metastable form. Themole fraction of CO₂ in the CO₂ nucleated water ranges from about 0.01to 0.04, usually from about 0.02 to 0.04, more usually from about 0.03to 0.04 The temperature of the CO₂ nucleated water will typically rangefrom about -1.5 to 10° C., preferably from about -1.5 to 5° C., and morepreferably from about -1.5 to 0° C.

CO₂ nucleated water employed in the subject methods as the selectiveliquid solvent may be prepared using any convenient means. Oneconvenient means of obtaining CO₂ nucleated water is described in U.S.Application Ser. No. 08/291,593, filed Aug. 16, 1994, now U.S. Pat. No.5,562,891, the disclosure of which is herein incorporated by reference.In this method CO₂ is first dissolved in water using any convenientmeans, e.g. bubbling a stream of CO₂ gas through the water, injection ofCO₂ into the water under conditions of sufficient mixing or agitation toprovide for homogeneous dispersion of the CO₂ throughout the water, andthe like, where the CO₂ source that is combined with the water in thisfirst stage may be either in liquid or gaseous phase. Where gaseous CO₂is combined with water to make the CO₂ nucleated water, the gaseous CO₂will typically be pressurized, usually to partial pressures rangingbetween 6 to 100 atm, more usually between about 10 to 30 atm. The CO₂may be derived from any convenient source. In a preferred embodiment, atleast a portion of the CO₂ is gaseous CO₂ obtained from a CO₂ hydrateslurry decomposition step, as described in greater detail below. Thewater in which the CO₂ is dissolved may be fresh water or salt water,e.g. sea water. The temperature of the water will generally range fromabout -1.5 to 10° C., usually from about -1.5 to 5° C., more usuallyfrom about 0 to 1° C.

The water that is used to produce the nucleated water may be obtainedfrom any convenient source, where convenient sources include the deepocean, deep fresh water aquifers, powerplant cooling ponds, and thelike, and cooled to the required reactor conditions. In certainembodiments, the nucleated water may be recycled from a downstreamsource, such a clathrate slurry heat exchanger/decomposition source (asdescribed in greater detail below) where such recycled nucleated watermay be supplemented as necessary with additional water, which water mayor may not be newly synthesized nucleated water as described above.

The amount of CO₂ which is dissolved in the water will be determined inview of the desired CO₂ mole fraction of the CO₂ nucleated water to becontacted with the gaseous stream. One means of obtaining CO₂ nucleatedwater having relatively high mole fractions of CO₂ is to produce aslurry of CO₂ clathrates and then decompose the clathrates by loweringthe pressure and/or raising the temperature of the water component ofthe slurry. Generally, nucleated water having higher mole fractions ofCO₂ are desired because it more readily accepts CO₂ absorption andexcludes formation of other hydrate compounds.

The production of CO₂ nucleated water may conveniently be carried out ina nucleation reactor. The reactor may be packed with a variety ofmaterials, where particular materials of interest are those whichpromote the formation of CO₂ nucleated water and include: stainlesssteel rings, carbon steel rings, and the like, to promote gas-liquidcontact. To ensure that the optimal temperature is maintained in thenucleation reactor, active coolant means may be employed. Any convenientcoolant means may be used, where the coolant means will typicallycomprise a coolant medium housed in a container which contacts thereactor, preferably with a large surface area of contact, such as coilsaround and/or within the reactor or at least a portion thereof, such asthe lower portion of the reactor. Coolant materials or media of interestinclude ammonia, HCFCs, and the like, where a particular coolantmaterial of interest is ammonia, where the ammonia is maintained at atemperature of from about 0 to 10° C. The surface of the cooling coils,or a portion thereof, may be coated with a catalyst material, such as anoxide of aluminum, iron, chromium, titanium, and the like, to accelerateCO₂ hydrate precursor formation. Additionally, hydrate crystal seedingor a small (1-3 atm) pressure swing may be utilized to enhance hydrateprecursor formation.

The first step of the subject method is to contact the multicomponentgaseous stream with CO₂ nucleated water under conditions of CO₂clathrate formation, preferably under conditions of selective CO₂clathrate formation. The CO₂ nucleated water may be contacted with thegaseous stream using any convenient means. Preferred means of contactingthe CO₂ nucleated water with the gaseous stream are those means thatprovide for efficient absorption of the CO₂ from the gas throughsolvation of the gaseous CO₂ in the liquid phase CO₂ nucleated water.Means that may be employed include concurrent contacting means, i.e.contact between unidirectionally flowing gaseous and liquid phasestreams, countercurrent means, i.e. contact between oppositely flowinggaseous and liquid phase streams, and the like. Thus, contact may beaccomplished through use of spray, tray, or packed column reactors, andthe like, as may be convenient.

Generally, contact between the multicomponent gaseous stream and thenucleated water is carried out in a hydrate or clathrate formationreactor. The reactor may be fabricated from a variety of materials,where particular materials of interest are those which promote theformation of CO₂ clathrates or hydrates and include: stainless steel,carbon steel, and the like. The reactor surface, or a portion thereof,may be coated with a catalyst material, such as an oxide of aluminum,iron, chromium, titanium, and the like, to accelerate CO₂ hydrateformation. To ensure that the optimal temperature is maintained in thehydrate formation reactor, active coolant means may be employed. Anyconvenient coolant means may be used, where the coolant means willtypically comprise a coolant medium housed in a container which contactsthe reactor, preferably with a large surface area of contact, such ascoils around or within the reactor or at least a portion thereof, suchas the exit plenum of the reactor. Coolant materials or media ofinterest include ammonia, HCFCs and the like, where a particular coolantmaterial of interest is ammonia, where the ammonia is maintained at atemperature of from about 0 to 10° C. Where the reactor comprises gasinjectors as the means for achieving contact to produce clathrates, thereactor may comprise 1 or a plurality of such injectors. In suchreactors, the number of injectors will range from 1 to about 20 or more,where multiple injectors provide for more rapid clathrate production.

The clathrate formation conditions under which the gaseous and liquidphase streams are contacted, particularly the temperature and pressure,may vary but will preferably be selected so as to provide for theselective formation of CO₂ clathrates, to the exclusion of clathrateformation of other components of the multi-component gaseous stream.Generally, the temperature at which the gaseous and liquid phases arecontacted will range from about -1.5 to 10° C., usually from about -1.5to 5° C., more usually from about 0 to 1° C. The CO₂ partial pressure orthe total pressure in the reactor will generally be at least about 6atm, usually at least about 8 atm, and more usually at least about 10atm, but will generally not exceed 100 atm, and more usually will notexceed 30 atm, where higher pressures are required when highertemperatures are employed, and vice versa.

Upon contact of the gaseous stream with the CO₂ nucleated water, CO₂ isselectively absorbed from the gaseous stream into the CO₂ nucleatedwater liquid phase. The absorbed CO₂ is concomitantly fixed as solid CO₂clathrates in the liquid phase. Contact between the gaseous and liquidphases results in the production of a CO₂ depleted multicomponentgaseous stream and a slurry of CO₂ clathrates. In the CO₂ depletedmulticomponent gaseous stream, the CO₂ concentration is reduced by atleast about 50%, usually by at least about 70%, and more usually by atleast about 90%, as compared to the untreated multicomponent gaseousstream. In other words, contact of the multicomponent gaseous streamwith the CO₂ nucleated water results in at least a decrease in theconcentration of the CO₂ of the gaseous phase, where the decrease willbe at least about 50%, usually at least about 70%, more usually at leastabout 90%. In some instances the concentration of CO₂ in the gaseousphase may be reduced to the level where it does not exceed 1% (v/v),such that the treated gaseous stream is effectively free of CO₂ solutegas.

As discussed above, the CO₂ absorbed by the CO₂ nucleated water isconcomitantly fixed in the form of stable CO₂ clathrates. Fixation ofthe CO₂ in the form of stable CO₂ clathrates results in the conversionof the CO₂ nucleated water to a slurry of CO₂ clathrates. The slurry ofCO₂ clathrates produced upon contact of the gaseous stream with the CO₂nucleated water comprises CO₂ stably fixed in the form of CO₂ clathratesand water. Typical mole fractions of CO₂ in stable clathrates are 0.12to 0.15, as compared to 0.02 to 0.04 in the CO₂ nucleated water.

As described above, the CO₂ nucleated water that serves as the selectiveliquid solvent for the CO₂ solute gas of the multicomponent gaseousstream is produced by dissolving CO₂ in water. As such, in someembodiments of the subject invention, CO₂ free water may be contactedwith the multicomponent gaseous stream under appropriate conditions tofirst produce the CO₂ nucleated water, where contact will besubsequently maintained to produce the CO₂ clathrate slurry. In otherwords, the separate steps of CO₂ nucleated water production and thecontact between the gaseous stream and the CO₂ nucleated water arecombined into one continuous process.

The second step of the subject method is the separation of the treatedgaseous phase from the CO₂ clathrate slurry. As convenient, the gaseousphase may be separated from the slurry in the reactor or in a downstreamgas-liquid separator. Any convenient gas-liquid phase separation meansmay be employed, where a number of such means are known in the art.

Where it is desired to sequester the CO₂ clathrates produced by thesubject method, the resultant CO₂ clathrate slurry may be disposed ofdirectly as is known in the art, e.g. through placement in gas wells,the deep ocean or freshwater aquifers, and the like, or subsequentlyprocessed to separate the clathrates from the remaining nucleated water,where the isolated clathrates may then be disposed of according tomethods known in the art and the remaining nucleated water recycled forfurther use as a selective CO₂ absorbent in the subject methods, and thelike. Where desired, CO₂ can easily be regenerated from the clathrates,e.g. where CO₂ is to be a product, using known methods. The resultantCO₂ gas may be disposed of by transport to the deep ocean or groundaquifers, or used in a variety of processes, e.g. enhanced oil recovery,coal bed methane recovery, or further processed to form metalcarbonates, e.g. MgCO₃, for fixation and sequestration.

In a preferred embodiment, the CO₂ hydrate slurry is treated in a mannersufficient to decompose the hydrate slurry into CO₂ gas and CO₂nucleated water, i.e. it is subjected to a decomposition step.Typically, the CO₂ hydrate slurry is thermally treated, e.g. flashed,where by thermally treated is meant that temperature of the CO₂ hydrateslurry is raised in sufficient magnitude to decompose the hydrates andproduce CO₂ gas. One convenient means of thermally treating the CO₂hydrate slurry is in a counterflow heat exchanger, where the heatexchanger comprises a heating medium in a containment means thatprovides for optimal surface area contact with the clathrate slurry. Anyconvenient heating medium may be employed, where specific heating mediaof interest include: ammonia, HCFC's and the like, with ammonia vapor ata temperature ranging from 20 to 40° C. being of particular interest.Preferably, the ammonia vapor is that vapor produced in cooling thenucleation and/or hydrate formation reactors, as described in greaterdetail in terms of the figures.

A variety of multicomponent gaseous streams are amenable to treatmentaccording to the subject methods. Multicomponent gaseous streams thatmay be treated according to the subject invention will comprise at leasttwo different gaseous components and may comprise five or more differentgaseous components, where at least one of the gaseous components will beCO₂, where the other component or components may be one or more of N₂,O₂, H₂ O, CH₄, H₂, CO and the like, as well as one or more trace gases,e.g. H₂ S. The total pressure of the gas will generally be at leastabout 5 atm, usually at least about 6 atm and more usually at leastabout 10 atm. The mole fraction of CO₂ in the multicomponent gaseousstreams amenable to treatment according to the subject invention willtypically range from about 0.05 to 0.65, usually from about .10 to 0.60,more usually from about 0.10 to 0.50. Generally, the partial pressure ofCO₂ in the multicomponent gaseous stream will be at least about 4 to 6atm, where in many embodiments the partial pressure of the CO₂ will beleast about 10 atm and as great as 40 atm. As mentioned above, bycontrolling the clathrate formation conditions of contact appropriately,contact between the CO₂ nucleated water and the gas can be controlled toprovide for the selective formation of CO₂ clathrates, e.g. through useof highly nucleated water that selectively absorbs CO₂ gas, such asnucleated water produced through the introduction of pure CO₂ gas intothe nucleation reactor. The particular conditions which provide for thebest selectivity with a particular gas can readily be determinedempirically by those of skill in the art. Particular multicomponentgaseous streams of interest that may be treated according to the subjectinvention include power plant flue gas, turbo charged boiler productgas, coal gasification product gas, shifted coal gasification productgas, anaerobic digester product gas, wellhead natural gas and the like.

Generally, the partial pressure of each of the components of themulticomponent gaseous medium will be such that CO₂ is selectivelyabsorbed by the nucleated water and other components are not. As such,the partial pressure of CO₂ in the multicomponent gaseous stream will besufficiently high and the partial pressure of each of the othercomponents of the multicomponent gaseous stream will be sufficiently lowto provide for the desired selective CO₂ absorption.

Multicomponent gaseous mediums in which the partial pressures of each ofthe components are suitable for selective CO₂ hydrate formationaccording to the subject invention may be treated directly without anypretreatment or processing. For those multicomponent gaseous mediumsthat are not readily suitable for treatment by the subject invention,e.g. in which the partial pressure of CO₂ is too low and/or the partialpressure of the other components are too high, may be subjected to apretreatment or preprocessing step in order to modulate thecharacteristics of the gaseous medium so that is suitable for treatmentby the subject method. Illustrative pretreatment or preprocessing stepsinclude: temperature modulation, e.g. heating or cooling, decompression,incorporation of additional components, e.g. CO₂, and the like.

One particular multicomponent gas of interest that may be treatedaccording to the subject invention is natural well head gas, e.g.natural gas comprising one or more lower akyls, such as methane, ethane,butane and the like. Where the gas conditions are appropriate, CO₂ maybe separated from the gas according to the subject invention directly atthe well head site without modification or processing of the gas. Forexample, if the wellhead gas has a total pressure of approximately 60 to70 atm, a temperature of 0 to 5° C. and consists substantially of CO₂and methane, where the amount of CO₂ present in the gas is greater thanabout 50 volume %, the wellhead gas can be treated without modificationaccording to the subject invention. Conversely, where the temperature ofthe well head gas is closer to 10° C., as long as the partial pressureof CO₂ in the methane/CO₂ mixture is at least 30 atm and the partialpressure of methane is below 60 atm, the wellhead gas can be processedwithout pretreatment. Where the well head gas conditions are notdirectly suitable for treatment, the wellhead gas may be processed tomake it suitable for treatment as described above, where processingincludes temperature modulation, e.g. heating or cooling, decompression,incorporation of additional components, e.g. CO₂, and the like. Forexample, where the concentration of CO₂ in the wellhead gas is fromabout 15 to 50% and the temperature is from 0 to 5° C., the gas streamcan be treated by decompressing it in a manner sufficient to maintainthe partial pressure of the CO₂ component above 10 atm and achieve apartial pressure of methane that is below 20 atm.

For the treatment of a shifted coal gasification product gas, theuntreated gas will typically comprise H₂, H₂ O, CO₂ and trace gases,where the mole fraction of CO₂ will range from about 0.30 to 0.45, andwill be at a temperature ranging from about 20 to 30° C. and a pressureranging from about 20 to 40 atm. The product gas will first be cooled toa temperature ranging from -30 to 0° C. and then contacted with CO₂nucleated water, as described above, to produce CO₂ depleted shiftedcoal gasification product gas and a slurry of CO₂ clathrates. Theresultant CO₂ depleted product gas and CO₂ clathrate slurry may then beseparated and the product gas used as a fuel or for chemical synthesis.Where the shifted coal gas comprises trace amounts of H₂ S, H₂ S willgenerally be present in amounts ranging from 0 to 20 mole percent or 0to 3.6 weight percent. In such cases, the H₂ S will either be dissolvedin the excess slurry water or will form hydrates along with the CO₂ andtherefore be separated from the multicomponent gaseous stream, obviatingthe need for subsequent H₂ S removal steps.

Yet another type of multicomponent gas that is particularly suited fortreatment according to the subject invention is powerplant gas producedby the use of pure oxygen instead of air in the combustion of organicfuels, such as coal, oil or natural gas. In a preferred embodiment, thispowerplant flue gas is compressed to 4 to 20 atm and contacted with CO₂nucleated water. The resultant hydrate slurry is nearly pure and is thenpressurized to 100 to 150 atm, which can then be flashed as describedabove to produce CO₂ gas and nucleated water, where the CO₂ gas can thenbe disposed of or utilized, e.g. in coal bed methane recovery orenhanced oil recovery.

The invention will now be described further in terms of the figures,which provide schematic representations of countercurrent and concurrentreactors for carrying out the subject invention. FIG. 1 provides aschematic representation of an embodiment of a counter current reactorwhich may be used for carrying out the subject process. Reactor 10comprises counter current gaseous-liquid phase contact region 12surrounded by refrigerant chamber 26, which serves to keep thetemperature of region 12 suitable for selective CO₂ clathrate formation.Multicomponent gaseous stream 16 comprising CO₂ enters region 12 whereit is contacted with CO₂ nucleated water, or non-nucleated water if thethe CO₂ nucleated water is to be formed in situ, from feed stream 14.The region may have an open structure, i.e. where the nucleated water issprayed in countercurrent to the gas, a trayed structure or a packedstructure, as is known in the art. Fresh refrigerant liquid or coolantmedium is introduced into refrigerant chamber 26 by feed stream 22 andspent refrigerant, represented by stream 24, is returned to refrigerantsystem 20, where it is refreshed and cycled back to chamber 26. Treatedgas, represented by stream 30,leaves chamber 12 and may be recycledthrough line 28 or passed through a liquid, gas phase separator 32 toyield dry product gas, represented by stream 36, and water, representedby stream 34. The resultant slurry of CO₂ clathrates and nucleatedwater, represented by stream 18, may be sequestered using any convenientmeans, as described above.

FIG. 2 provides a schematic representation of a concurrent reactor forcarrying out the subject invention, in which the gaseous stream andliquid phase are contacted in unidirectional flow. With concurrentreactor 40, multicomponent gaseous stream 42 and liquid CO₂ nucleatedwater stream 44 are introduced separately into the reactor and flow to acombination region 52 where they then travel together to a region ofselective CO₂ clathrate formation cooled by refrigerant coils 46. Theresultant slurry of CO₂ clathrates, represented by stream 48, isseparated from the CO₂ depleted gaseous stream 50. The CO₂ clathrateslurry 48 may then be sequestered as described above, while the CO₂depleted gaseous stream 50 may be further processed in a liquid-gasphase separator, as described above.

FIG. 3 provides a schematic flow diagram of a preferred embodiment ofthe subject method in which parasitic energy losses are reduced to aminimum. In FIG. 3, the multicomponent gaseous stream and CO₂ nucleatedwater are combined in the CO₂ hydrate reactor under conditionssufficient to produce a CO₂ hydrate slurry and a CO₂ depletedmulticomponent gaseous stream. The reactor is cooled with an ammoniacoolant which vaporizes in the cooling process. The ammonia vapor isfurther compressed and the ammonia flows to a condenser or a CO₂clathrate heat exchanger. The CO₂ hydrate slurry and other gases arethen separated in the slurry/gas separator. The slurry is then sent tothe CO₂ clathrate heat exchanger where the slurry is thermally treatedto produce CO₂ gas and nucleated water and condensed ammonia liquid.Preferably, the compressed ammonia vapor produced from cooling the CO₂hydrate reactor is used to thermally treat, at least in part, the CO₂hydrate slurry in the heat exchanger, as this greatly reduces parasiticenergy loss. The resultant CO₂ nucleated water is then chilled, e.g.using cooled amonia, and returned to the nucleation reactor, where it iscombined with chilled makeup water and/or a recycled stream of pure CO₂(which may be produced from the heat exchanger exhaust CO₂) to make CO₂nucleated water of sufficiently high CO₂ content to selectively removeCO₂ from additional multicomponent gas. The CO₂ gas produced from theheat exchanger can be disposed of, e.g. by deposition in the deep oceanor ground aquifers or used in subsequent processes, e.g. enhanced oilrecovery or coal bead methane recovery. By cycling the coolant medium(ammonia) between the heat exchanger and at least one of the coolingcoils of the nucleation and hydrate formation reactors and recycling thenucleated water produced in the CO₂ clathrate heat exchanger, parasiticenergy losses and resource use are minimized.

FIG. 4 provides a detailed schematic flow diagram of the CO₂ heatexchange component of the process depicted in FIG. 3.

The following examples are offered by way of illustration and not by wayof limitation.

EXPERIMENTAL

The following experiments demonstrate the viability of reducing the CO₂content of multicomponent gaseous streams by directly contacting thestreams with CO₂ nucleated water under conditions of selective CO₂clathrate formation. The following gaseous streams are representative ofmulticomponent gases suitable for treatment with the subject methods.

    ______________________________________                                                 Initial                     Approximate                                 Temp. Initial Pressure  CO.sub.2 Mole                                        Gas Stream (° C.) (ATM, gauge) Components Fraction                   ______________________________________                                        Power Plant                                                                            80-150  0.1-0.2    N.sub.2, O.sub.2, H.sub.2 O,                                                           0.10-0.15                                  Flue Gas   CO.sub.2, & Trace                                                     Gases                                                                      Turbo Charged 80-150 10-20 N.sub.2, O.sub.2, H.sub.2 O, 0.10-0.15                                                 Boiler   CO.sub.2, & Trace                   Gases                                                                      Coal 20-30  20-40 H.sub.2, CO, H.sub.2 O, 0.02-.12                            Gasification   CO.sub.2, & Trace                                              Product Gas   Gases                                                           Shifted Coal 20-30  20-40 H.sub.2, H.sub.2 O, CO.sub.2, 0.30-0.45                                                 Gasification   & Trace Gases                                                  Product Gas                               Anaerobic 20-40    0-0.2 CH.sub.4, CO.sub.2 0.40-0.50                         Digester                                                                      Product Gas                                                                   Wellhead 20-100 50-80 CH.sub.4, CO.sub.2, 0.03-0.90                           Natural   Higher                                                              Gas   Hydrocarbons                                                            Fossil Fuel 80-150 0.1-0.2 CO.sub.2 & Trace 90-99                             Combustion   Gases                                                            with Pure                                                                     Oxygen                                                                      ______________________________________                                    

I. Power Plant Flue Gas

Flue gas from a conventional coal fired boiler is processed to removesulfur oxides, some of the nitrogen oxides and any particulate matter.The processed flue gas is then compressed to approximately 20-50 atm.The flue gas is then contacted with CO₂ in the counter current reactorshown in FIG. 1, where the temperature in the reactor is maintained at0.5° C. and the pressure is maintained at approximately 20-50 atm.

The flue gas is scrubbed to form a slurry of CO₂ clathrates and CO₂ freegas. The CO₂ free gas, comprising N₂, O₂, H₂ O and trace gases is thenseparated from the CO₂ clathrate slurry, and reheated to the requiredstack gas exhaust temperature. The CO₂ clathrate slurry is sequesteredthrough deposition in the deep ocean or other appropriate repository.Since modest pressure levels (20-50 atm) are maintained in the reactor,minimal amounts of nitrogen or oxygen clathrates form during theprocess.

II. Turbo Charged Boiler Emission Gas

Coal is combusted in a turbo charged boiler. The exhaust gas, containingnitrogen (N₂), oxygen (O₂), water vapor, carbon dioxide and trace gasesis cooled to 0° C. The cooled, clean exhaust gas is then furthercompressed to 20 to 50 atm and contacted with CO₂ nucleated water in thereactor shown in FIG. 1. The pressure inside the reactor isapproximately 20 to 50 atm while the temperature is maintained at 0° C.Contact in the reactor results in the production of CO₂ free gas and aslurry comprising CO₂ clathrates. The CO₂ free exhaust gas stream isthen reheated using a recuperative heat exchanger expanded through anexpansion turbine and exhausted to the atmosphere. The CO₂ clathrateslurry is sequestered through deposition in the deep ocean or otherappropriate repository.

Since the reactor conditions are controlled to 20 to 50 atm, very littlenitrogen or oxygen clathrates are formed.

III. Coal Gasification Product Gas

Syngas is produced through gasification of coal with nearly pure oxygen.The resultant syngas comprises hydrogen, carbon monoxide, water vapor,carbon dioxide, and trace gases. Following removal of the trace gases,the syngas (which is at 20 to 40 atm) is cooled to 0° C. and contactedwith CO₂ nucleated water in the countercurrent reactor shown in FIG. 1.Contact of the syngas with the CO₂ nucleated water results in theproduction of a slurry of CO₂ clathrates and a CO₂ free syngas, whichare then separated. The resultant CO₂ free syngas may be utilized as achemical feedstock or fuel.

IV. Steam Shifted Coal Gasification Product Gas

Prior to contact with CO₂ nucleated water, synthesis gas produced inaccordance with Example III above is steam shifted to produce a gasconsisting essentially of CO₂ and H₂. The shifted gas is then contactedwith CO₂ nucleated water in the countercurrent reactor shown in FIG. 1,where the system pressure is 20 to 50 atm and the temperature is 0° C.Contact between the shifted synthesis gas and the CO₂ nucleated waterresults in the fixation of essentially 100% of the CO₂ component of theshifted synthesis gas as CO₂ clathrates, since the hydrogen does notform stable clathrates. The resultant slurry comprising the CO₂clathrates is then sequestered by deposition in the deep ocean or otherappropriate repository. The treated syngas is an essentially purehydrogen stream which is further used in power production, as refineryhydrogen or in chemical synthesis.

V. Anaerobic Digester Product Gas

Product gas comprising 50-60% CH₄ and 50-40% CO₂, as well as certaintrace gases such as ammonia and hydrogen sulfide, is obtained from theanaerobic digestion of sewage sludge, wastes, mircoalgae or macroalgae.The product gas is at atmospheric pressure.

The product gas is compressed to approximately 10-30 atm and combinedwith CO₂ nucleated water in the countercurrent reactor shown in FIG. 1.The pressure in the reactor is approximately 10-30 atm and thetemperature is 0° C. Contact between the product gas and the CO₂nucleated water results in fixation of 100% of the CO₂ of the productgas as clathrates, leaving an essentially pure methane stream. The CO₂clathrate comprising slurry is deposited in the ocean or otherappropriate repository, as described above, while the pure methanestream is condensed to produce liquefied natural gas.

VI. Wellhead Natural Gas

Wellhead natural gas comprising CH₄ and CO₂, where the amount of CO₂exceeds 50% is processed directly at the wellhead. The wellhead naturalgas is at 60 to 80 atm and 0-5° C.

The well head natural gas is processed in the reactor shown in FIG. 3.Contact between the wellhead natural gas and the CO₂ nucleated waterresults in fixation of 100% of the CO₂ of the product gas as clathrates,leaving an essentially pure methane stream. The CO₂ clathrate comprisingslurry is deposited in the ocean or other appropriate repository, asdescribed above, while the pure methane stream is routed to a pipelinefor distribution.

For wellhead natural gas in which the partial pressure of CO₂ is between15 and 50%, the gas stream is first decompressed prior to processing inthe reactor of FIG. 3. In decompressing the gas stream, the partialpressure of CO₂ is engineered to be above 10 atm while the partialpressure of CH₄ is engineered to be below 20 atm. The temperature ismaintained at 0 to 5° C. To keep parasitic energy loss to a minimum, thegas stream may be decompressed in an expansion turbine and the energygenerated therefrom used to recompress the product methane gas.

To process the gas according to the subject invention in which thepartial pressure of CO₂ in the gas is less than 15%, the characteristicsof the nucleated water are selected to provide for optimal selectiveabsorption of CO₂ over other hydrocarbons present in the wellheadnatural gas. For example, hydrates could be introduced into thenucleated water prior to contact, promoting selective absorption of CO₂from the wellhead gas.

VII. Gas Produced By Combustion of Fossil Fuel with Pure Oxygen

Fossil fuel is combusted using pure oxygen. The resultant flue gascomprises CO₂ and trace gases, and has an initial pressure of 0.1 to 0.2atm.

The flue gas is compressed to 4 to 20 atm and then processed in thereactor shown in FIG. 3. Following CO₂ extraction, the nearly pure CO₂hydrate slurry is pressurized to 100 to 150 atm in a reciprocatingcharge pump or comparable pump system. The high pressure stream is thenflashed to free the high pressure CO₂ for transport to the deep ocean orground aquifers or to be utilized for coal bed methane recovery orenhanced oil recovery.

VIII. Concurrent Reactor

Each of the above treatments in Examples I through V are carried out ina concurrent reactor as shown in FIG. 2.

It is evident from the above results that a simple and efficient methodfor the selective removal of CO₂ from a multicomponent gaseous stream isprovided. By using CO₂ nucleated water as a selective CO₂ absorbent,great efficiencies are achieved through reductions in the overall energyinput requirements and the number of steps necessary for complete CO₂removal, fixation and disposal. In particular, by using CO₂ nucleatedwater as the absorbent solvent, CO₂ is readily removed from the gaseousstream and immediately fixed in a form suitable for disposal. Bytreating the resultant CO₂ hydrate slurry to produce CO₂ gas andnucleated water, even further reductions in parasitic energy loss areobtained, where such reductions stem from the use of recycled nucleatedwater, recycled coolant medium, e.g. amonia, and the like.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be readily apparent to those of ordinary skill inthe art in light of the teachings of this invention that certain changesand modifications may be made thereto without departing from the spiritor scope of the appended claims.

What is claimed is:
 1. A method for removing CO₂ from a multicomponentgaseous stream to produce a CO₂ depleted gaseous stream, said methodcomprising:contacting said multicomponent gaseous stream with CO₂nucleated water comprising water produced by decomposition of a CO₂clathrate slurry under conditions of CO₂ clathrate formation, wherebyCO₂ is absorbed from said gaseous stream by said CO₂ nucleated water andconcomitantly fixed as CO₂ clathrates upon said contacting, whereby aCO₂ depleted gaseous stream and a CO₂ clathrate slurry are produced;separating said CO₂ depleted gaseous stream from said CO₂ clathrateslurry; and decomposing said CO₂ clathrate slurry to produce CO₂ gas andCO₂ nucleated water.
 2. The method according to claim 1, wherein saidmulticomponent gaseous stream has a CO₂ partial pressure of at leastabout 5 atm.
 3. The method according to claim 1, wherein said CO₂clathrate decomposition step comprises heating said CO₂ clathrateslurry.
 4. A method for selectively removing CO₂ from a multicomponentgaseous stream to produce a CO₂ depleted gaseous stream, said methodcomprising:preparing CO₂ nucleated water by contacting CO₂ gas withwater in a nucleation reactor; contacting said multicomponent gaseousstream with said CO₂ nucleated water in a CO₂ hydrate formation reactorunder conditions of selective CO₂ clathrate formation, whereby CO₂ isabsorbed from said gaseous stream by said CO₂ nucleated water andconcomitantly fixed as CO₂ clathrates upon said contacting, whereby aCO₂ depleted gaseous stream and a CO₂ clathrate slurry are produced;separating said CO₂ depleted gaseous stream from said CO₂ clathrateslurry; and heating said CO₂ clathrate slurry in a manner sufficient toproduce CO₂ gas and CO₂ nucleated water.
 5. The method according toclaim 4, wherein said CO₂ nucleated water production step comprisescombining CO₂ nucleated water produced by heating said CO₂ clathrateslurry with additional make up water and CO₂ gas.
 6. The methodaccording to claim 4, wherein at least one of said nucleation reactorand hydrate production reactor are cooled with ammonia at a temperatureof from about 0 to 10° C.
 7. The method according to claim 4, whereinsaid heating step comprises processing said CO₂ clathrate slurry in aheat exchanger with ammonia at a temperature of from about 20 to 40° C.8. The method according to claim 7, wherein said multicomponent gaseousstream is selected from the group of multicomponent gaseous streamsconsisting of natural gas, power plant flue gas, turbo charged boileremission gas, coal gasification product gas, shifted coal gasificationproduct gas, anaerobic digester product gas, wellhead natural gas andflue gas produced through combustion of fossil fuel with pure oxygen. 9.The method according to claim 4, wherein said hydrate formation reactorcomprises from 1 to 20 multicomponent gas injectors.
 10. A method forselectively removing CO₂ from a multicomponent gaseous stream to producea CO₂ depleted gaseous stream, said method comprising:preparing CO₂nucleated water by contacting CO₂ gas with water in a nucleationreactor, wherein said CO₂ nucleated water is prepared by combiningrecycled nucleated water with CO₂ gas and make up water and said reactoris cooled with a cooling means comprising ammonia at a temperature offrom 0 to 10° C.; contacting said multicomponent gaseous stream withsaid CO₂ nucleated water in a CO₂ hydrate formation reactor underconditions of selective CO₂ clathrate formation, wherein said contact isachieved by injecting said multicomponent gaseous stream into saidreactor through 1 to 20 injectors, whereby CO₂ is absorbed from saidgaseous stream by said CO₂ nucleated water and concomitantly fixed asCO₂ clathrates upon said contacting, whereby a CO₂ depleted gaseousstream and a CO₂ clathrate slurry are produced; separating said CO₂depleted gaseous stream from said CO₂ clathrate slurry; and decomposingsaid CO₂ clathrate slurry in a heat exchanger in a manner sufficient toproduce CO₂ gas and CO₂ nucleated water.
 11. The method according toclaim 10, wherein said heat exchanger comprises ammonia at a temperatureof from about 20 to 40° C.